Late Front Caliper Piston, Wont Compress Back In Options

[Costa05]

Back story. Car has not been driven for probably 20 years but getting close to getting back on the road again. These are late front calipers by the way. I removed the calipers to clean things up and put some new hardware and brake pads on. Picked up some almost new rotors those are good. Tried to compress the caliper pistons back in with the calipers removed from the car and would not budge using 6" C clamp and wood protector on piston (lots of force by the way). Figured they were frozen so I did the grease gun trick to try to force the pistons to move out which worked almost effortlessly (inner and outer piston). Before forcing them out all the way I tried to push them back in to get movement both ways and got nothing. Neither side will budge trying to compress them back in. Bleeders are removed. No brake lines attached to it at all. Thought I would get grease pushing as the pistons got forced went back in. I am stumped. How could they be frozen in one direction and not the other?

[Chris914n6]

the seals are positioned so that they are beveled. lube the piston should help but at 20 years it's really time for new seals.

[PatMc]

Back story. Car has not been driven for probably 20 years but getting close to getting back on the road again. These are late front calipers by the way. I removed the calipers to clean things up and put some new hardware and brake pads on. Picked up some almost new rotors those are good. Tried to compress the caliper pistons back in with the calipers removed from the car and would not budge using 6" C clamp and wood protector on piston (lots of force by the way). Figured they were frozen so I did the grease gun trick to try to force the pistons to move out which worked almost effortlessly (inner and outer piston). Before forcing them out all the way I tried to push them back in to get movement both ways and got nothing. Neither side will budge trying to compress them back in. Bleeders are removed. No brake lines attached to it at all. Thought I would get grease pushing as the pistons got forced went back in. I am stumped. How could they be frozen in one direction and not the other?

I'll add that it's a good thing they won't go back in after you've filled the caliper with grease. Brake systems use EPDM rubber seals, which don't get along well with petroleum based oils/greases. The calipers need to be disassembled and thoroughly cleaned...which will be a bitch because nothing really dissolves new grease very well.

The seals are already junk...including the crossover seals. Clean everything very well at least twice, then go back together with all new rubber using the appropriate assembly lube.

[Shivers]

Back story. Car has not been driven for probably 20 years but getting close to getting back on the road again. These are late front calipers by the way. I removed the calipers to clean things up and put some new hardware and brake pads on. Picked up some almost new rotors those are good. Tried to compress the caliper pistons back in with the calipers removed from the car and would not budge using 6" C clamp and wood protector on piston (lots of force by the way). Figured they were frozen so I did the grease gun trick to try to force the pistons to move out which worked almost effortlessly (inner and outer piston). Before forcing them out all the way I tried to push them back in to get movement both ways and got nothing. Neither side will budge trying to compress them back in. Bleeders are removed. No brake lines attached to it at all. Thought I would get grease pushing as the pistons got forced went back in. I am stumped. How could they be frozen in one direction and not the other?

I'll add that it's a good thing they won't go back in after you've filled the caliper with grease. Brake systems use EPDM rubber seals, which don't get along well with petroleum based oils/greases. The calipers need to be disassembled and thoroughly cleaned...which will be a bitch because nothing really dissolves new grease very well.

The seals are already junk...including the crossover seals. Clean everything very well at least twice, then go back together with all new rubber using the appropriate assembly lube.

Agreed, seals will break down from petroleum products. Best solvent I found for grease of any kind is lacquer thinner. Wear gloves

Brake fluid loves water, if not changed on a semi regular basis, it will corrode the system. Could be the pistons are bound up by corrosion.

[Costa05]

Back story. Car has not been driven for probably 20 years but getting close to getting back on the road again. These are late front calipers by the way. I removed the calipers to clean things up and put some new hardware and brake pads on. Picked up some almost new rotors those are good. Tried to compress the caliper pistons back in with the calipers removed from the car and would not budge using 6" C clamp and wood protector on piston (lots of force by the way). Figured they were frozen so I did the grease gun trick to try to force the pistons to move out which worked almost effortlessly (inner and outer piston). Before forcing them out all the way I tried to push them back in to get movement both ways and got nothing. Neither side will budge trying to compress them back in. Bleeders are removed. No brake lines attached to it at all. Thought I would get grease pushing as the pistons got forced went back in. I am stumped. How could they be frozen in one direction and not the other?

I'll add that it's a good thing they won't go back in after you've filled the caliper with grease. Brake systems use EPDM rubber seals, which don't get along well with petroleum based oils/greases. The calipers need to be disassembled and thoroughly cleaned...which will be a bitch because nothing really dissolves new grease very well.

The seals are already junk...including the crossover seals. Clean everything very well at least twice, then go back together with all new rubber using the appropriate assembly lube.

Agreed, seals will break down from petroleum products. Best solvent I found for grease of any kind is lacquer thinner. Wear gloves

Brake fluid loves water, if not changed on a semi regular basis, it will corrode the system. Could be the pistons are bound up by corrosion.

@shivers you were exactly right. Opened one up and sure enough the b piston sleeve was corroded on the inside lower section. Looked like the swelled up inside slightly. Will be sending them to PMB for rebuilds next week. Not sure if they are any good.